In papermaking, the use of shoe presses is on the increase because they contribute to a reduction in the total manufacturing cost. Furthermore, there is a trend toward the use of a closed type shoe press because it requires less space and avoids scattering of oil.
Compared to conventional belts used in open type shoe presses, belts used in a closed type shoe press are subject to more severe conditions, especially in terms of papermaking speed and nip pressure. Accordingly there has been a strong demand by users for improvement in belt durability.
Among typical technologies used for producing belts for closed type shoe presses, various manufacturing technologies using mandrels are known. For example, Examined Japanese Patent Publication No. 57236/1991, and Unexamined Japanese Patent Publication No. 45888/1989, disclose a manufacturing method using an endless woven fabric as a core member. In addition, Japanese Patent No. 3213589 discloses a manufacturing method using an endless mesh for a core member. However, these manufacturing methods have deficiencies, especially difficulties encountered in adjusting the machine direction dimension of a belt being produced.
In addition, PCT Patent application No. 503315/1989, and Unexamined Japanese Patent Publication No. 209578/1996, disclose a manufacturing method wherein a woven fabric is not used. These manufacturing methods form threads in the axial direction of a mandrel at regular intervals around the entire circumference of the mandrel. However, it is difficult to position the threads substantially parallel to the axial direction of the mandrel, and to avoid loosening of the threads under tensile force. With these methods excessive time is required for forming the threads.
Unexamined Japanese Patent Publication No. 298292/1989, and PCT Patent application No. 505428/1993, disclose a manufacturing method wherein a mat-shaped fiber band or a woven fabric impregnated with uncured resin is wound in a helix and then cured. However, with these manufacturing methods, exfoliation can easily occur at joints of the helix.
FIGS. 10(a) and 10(b) show a manufacturing method for a conventional shoe press belt. An endless woven fabric C is arranged on two rolls A and B, and impregnated and coated on an external surface of the woven fabric C by a coating apparatus D to form a shoe side layer, which is then cured. After curing of the shoe side layer, the endless woven fabric C is removed from the rolls A and B, turned inside-out, and reset on the rolls with its original inner surface facing outward. The fabric is again impregnated and coated to form a wet paper web side layer. The wet paper web side layer is cured, its thickness is adjusted, and concave grooves G are formed in its outer surface to produce a belt 1 is obtained, as shown in FIG. 10(b).
The above-described conventional method had two principal deficiencies. First, in order to impregnate and coat the shoe side layer E on one surface of the endless woven fabric and the wet paper web side layer F on the other side, the belt needed to be reversed, and reversal caused distortion to occur inside the belt. Second, since the distortion that existed when weaving the endless woven fabric is released as the resin is cured. Release of this distortion results in instability of the form of the belt, enabling flapping of the belt to occur.
Japanese Patent No. 3408416, and Unexamined Japanese Patent Publication No. 303377/2000, disclose a manufacturing method wherein a first resin layer is formed on a mandrel followed by formation of a base body around the external circumference of the resin layer, and formation of another resin layer, which is connected with first resin layer through the base body. According to this manufacturing method, after forming the first resin layer, there is no need to grind or reverse the resin layer, and therefore manufacturing efficiency and productivity can be improved.
The shoe press belt manufactured according to the manufacturing method disclosed in the Japanese Patent No. 3408416 has relatively large undulations at the joints of the warp yarns and weft yarns in the woven fabric used as its base body. In the use of the belt, these undulations result in large stress concentration at the joints of the warp yarns and weft yarns, which can result in cracking of a resin layer, and impairment of the durability of the belt.
In the case of a manufacturing method disclosed in Unexamined Japanese Patent Publication No. 303377/2000, similarly to the methods disclosed in PCT Patent application No. 503315/1989 and Unexamined Japanese Patent Publication No. 209578/1996, threads have to be formed in the axial direction of the mandrel at regular intervals, and be distributed around the entire circumference of the mandrel. The need for this arrangement of threads causes manufacture of the belt to be very time consuming and labor intensive.
It is an object of the invention to address the above-described problems, and to provide a shoe press belt that exhibits high crack resistance, and that can be produced efficiently.